1. Field of the Invention
This invention relates to a device and method for pole splitting, which is typically used to increase the bandwidth and stability of amplifiers.
2. Description of the Related Art
The frequency response of amplifiers and other circuits is determined by a transfer function, which is completely specified in terms of its poles and zeros together with a multiplicative constant. The upper frequency response limit, or bandwidth, of an amplifier typically depends upon a dominant pole. Amplifier stability, that is, whether the amplifier will be subject to undesired oscillations, also depends upon the dominant pole.
One conventional method for frequency compensation to increase amplifier bandwidth and stability involves connecting a feedback capacitor to an output stage in the amplifier. This causes the pole formed at the input of the amplifier to shift to a lower frequency and thus become dominant, while the pole formed at the output of the amplifier is moved to a very high frequency and thus becomes unimportant. This shifting of poles is known as pole splitting.
Amplifiers are often components of integrated circuits (chips). Digital chip fabrication processes are often preferable to analog processes since digital processes are cheaper and can be developed faster. However, in digital processes it is difficult to create capacitors with high capacitance value in a small chip area. The conventional method of pole splitting using a feedback capacitor is typically implemented with poly capacitors, metal poly sandwiched capacitors, and other capacitor types, with relatively high capacitance value. These capacitors consume a very large, inefficient amount of chip area when fabricated with digital processes.
Sometimes a metal oxide semiconductor field effect transistor (MOSFET) is used as a feedback capacitor. The problem with this prior art scheme is that the capacitance of the MOSFET transistor varies widely with the input signal; the capacitance may drop to a value which is too low for effective frequency compensation. Additionally, an amplifier suffers from undesired oscillations when the compensation capacitance drops too low. What is needed is a pole splitting device which has low variation in compensation capacitance.
Therefore, objects of this invention include providing a device for pole splitting which has a high value of effective capacitance while using very little chip area, and low variation in compensation capacitance.